This invention relates to optical waveguide forming methods. More particularly, it concerns an improved method for forming preforms from which optical fibers having improved transmission characteristics may be drawn.
Optical fiber waveguides have found widespread application in data transmission systems because of their small size and high rate data transmission capabilities as compared to previously utilized systems. The optical fibers, which are manufactured from a glass-like silica material and have a finished diameter as low as 5-10 microns, typically include a light transmitting inner zone or core and an outer cladding layer that surrounds the core. The cladding typically has an index of refraction less than that of the core and functions to confine most of the propagated light to the core, although as much as 30% of the light in the smaller, single-mode optical fibers can be transmitted in the cladding immediately adjacent to the core. While the core and cladding oftentimes have respective circular cross-sections, the core can also have non-circular configurations and can include longitudinal channels as well as webs and similar structures.
Various processes have been developed for manufacturing optical fibers, but most utilize a preform from which the optical fiber is drawn. A preform typically has a diameter (e.g. 25 mm) several orders of magnitude larger than that of the finished fiber and a cross sectional configuration that is the same, although on a much larger scale, as that of the finished fiber. In drawing the fiber, one end of the preform is inserted into a furnace and heated to a sufficiently high temperature to heat-soften the material which can then be drawn into a fiber.
A variety of processes have been developed for manufacturing preforms. In one process, an inner member and an outer member are separately formed from a high-purity silica. In the simpliest preform organization, the inner member can take the form of a solid cylindrical rod and the outer member can take the form of a hollow tube. The inner member is inserted into the outer member and the two members heated until the outer member collapses about and joins with the inner member to define a preform from which an optical fiber can be drawn.
Optical fibers produced from inner and outer starting members have the advantage of a relatively sharp step change in the index of refraction at the core to cladding interface since the characteristics and physical dimensions of the separately formed starting members can be accurately controlled prior to their assembly and joining. Also, the use of separately formed components permits the convenient manufacture of preforms having non-circular cross-sections, including preforms in which the inner members include open channels and supporting webs. Forming preforms using separate inner and outer members has generally not been successful for low loss transmission fibers due to defects such as caused by particulate or air inclusions at the interface between the two parts. The presence of inclusions or other defects at the interface results in optical fibers having diminished optical transmission characteristics, particularly in the smaller diameter single-mode fibers where as much as 30% of the energy is propagated in the cladding.
In other processes for fabricating preforms, such as in the chemical vapor deposition (CVD) process, a starting member, such as a hollow tube, is coated, either on its interior or exterior surfaces, with high purity particulate silica reaction products. The particulate silica can be doped to increase or decrease its index of refraction, as appropriate, to define a coated tube which is consolidated and collapsed to define the preform. In contrast to the aforementioned technique using separate starting components, the various CVD techniques are not well-suited to the formation of preforms having relatively sharp changes in the index of refraction, particularly in those CVD processes where multiple layers are applied, or to the formation of preforms having non-circular configurations, channels, or web-like structures. Problems associated with foreign particle and air inclusions are minimized, however, since the CVD techniques usually apply particulate silica directly onto a supporting surface under carefully controlled conditions.
As also can be appreciated, a need exists for a method of fabricating preform that, like the procedure which utilizes separate components, allows for the convenient control of the optical and physical characteristics of the separate components that eventually form the core and cladding of the finished fiber and which provides for a sharp step change in the index of refraction between the core and cladding of the finished fiber, and, like the various CVD techniques, provides a "clean" interface between the core and cladding of the resulting fiber.